Add a fallback mechanism for missing meta tags

While using nutch, we faced the following issue: some web pages miss a "description" meta tag, but include an "og:description" meta (using the open graph protocol).

Here are two examples:

• http://imagenesdelavirgenmaria.com/17-imagenes-de-la-virgen-maria-de-guadalupe/
• http://mixcdsource.com/product/dj-arson-dj-sin-cerothe-hit-list-18-5-reggaeton-edition/

It would be nice to have a configurable list of fallback meta tags to use when the main meta tag is absent. Something that would allow us to specify, in the configuration, "when the 'description' meta is missing, use 'og:description', when 'title' is missing, use 'og:title', etc..." .